Measurement and control of asphaltene agglomeration in hydrocarbon Liquids

ABSTRACT

A method is provided for measuring the agglomerative state of asphaltenes in a process flow stream of oil by applying an acoustic signal to the oil, detecting the scattered acoustic energy and using this detected signal to determine the relative particle size distribution of the asphaltene particles in the oil and/or their state of agglomeration. A method for controlling the agglomerative state of the asphaltenes which is based on the acoustic measurement technique is also provided.

BACKGROUND OF THE INVENTION

(1) Field of the Invention

The present invention relates to asphaltene-containing liquidhydrocarbons and, more particularly, to measurement and/or control ofthe agglomeration of asphaltenes in hydrocarbon liquids.

(2) Description of the Related Art

Asphaltenes are organic heterocyclic macromolecules which occur in crudeoils. Under normal reservoir conditions, asphaltenes are usuallystabilized in the crude oil dispersion by maltenes and resins that arechemically compatible with asphaltenes, but that have much lowermolecular weight. Polar regions of the maltenes and resins surround theasphaltene while non-polar regions are attracted to the oil phase. Thus,these molecules act as surfactants and result in stabilizing theasphaltenes in the crude. However, changes in pressure, temperature orconcentration of the crude oil can alter the stability of the dispersionand increase the tendency of the asphaltenes to agglomerate into largerparticles. As these asphaltene agglomerates grow, so does their tendencyto precipitate.

Precipitation of asphaltenes in crude oil or in process streams of oilis economically costly because of lost production and maintenancerequired to clear blockages caused by the solid materials.

Various methods have been devised to minimize asphaltene precipitation.For example, pressure and temperature conditions can be maintained,chemical stabilizers may be added to mimic and to enhance thestabilizing affect of the natural resins and maltenes, or devices suchas magnetic flux assemblies described in U.S. Pat. No. 5,453,188 may beused. While methods that minimize asphaltene precipitation can result insignificant economies, they have been hampered by a lack of a method formeasuring and monitoring the agglomerative state of the asphaltenes in aparticular stream at a particular time. Without knowing theagglomerative state of the asphaltenes in the stream, it is unclear whenor how much to treat the liquids to prevent asphaltene precipitation.

Conventional methods for determining the size and concentration ofasphaltene particles in hydrocarbons, such as those described in U.S.Pat. No. 4,238,451, or in Standard Method IP 143/84, require sampling,transport to a laboratory and testing by precipitation and filtration,centrifugation, titration with a destabilizing solvent, or othertime-consuming techniques. Thus, these methods are not suitable forreal-time, on-line monitoring of agglomeration.

Although methods for testing for the size and concentration of particlesin optically clear streams have been modified and applied tohydrocarbons, many have been much less successful in crude oil and otherin-process oil streams due to fouling and opacity. For example, theoptical system of Yamazoe, et al., U.S. Pat. No. 4,843,247, measuresasphaltene content, but provides a washing means to remove the samplesolution from the optical probes each time a sample measurement iscarried out. Such washing requires more complex measuring devices andinfers that fouling over time may hinder the accuracy of the opticalmeasurement.

Direct centrifugation of crude measures the total amount of asphaltenepresent, but provides no information on degree of agglomeration of theparticles or their tendency to remain in a stable dispersion.

Non-optical tests have recently shown promise in measuring particlecharacteristics. Anfindsen et al., U.S. Pat. No. 5,420,040, correlatedthe precipitation of asphaltenes in oil with changes in conductance orcapacitance. However, this method requires transferring a sample of theliquid to be measured to a measuring cell and is not carried outon-line. Furthermore, the process is carried out stepwise and cannot bedone substantially instantaneously since a time delay is required toallow for the precipitation of asphaltenes to occur.

Behrman and Larson, MBAA Technical Quarterly, 24, 72-76 (1987), describeon-line monitoring of particles over 0.8 microns in brewery streams bythe use of an ultrasonic monitoring device. They use a piezoelectrictransducer to generate an acoustic signal and to detect acoustic energyresulting from scattering from particles in the liquid streams. Thedevice permitted on-line, real time measurement of particleconcentrations, but did not permit the measurement of particle sizes ora particle size distribution.

More recently, Lin, et al., "Neutron Scattering Characterization ofAsphaltene Particles", presented at ACS National Meeting, San Francisco,Calif., April 1997, reported the use of small-angle neutron scattering(SANS) to determine the size and concentration of asphaltene particlesin dilute solution in 1-methyl-naphthalene-D10. The study concentratedon small, "basic", asphaltene particles and reported that largerparticles, which might be important to macroscopic properties, could notbe measured by today's small angle scattering instruments and would bevery difficult, if not impossible, to measure with light scatteringmethods.

de Boer, et al., SPE Production & Facilities, pp.5-61, February (1995),report the investigation of asphalt precipitation in oils and describeusing back-scattered energy from an acoustic probe to sense asphalteneparticles. A multi-channel analyzer was used to sort the signals intotwo amplitude classes corresponding to small and large particle sizes.The acoustic sensing procedure was used to monitor the relative numbersof large and small particles during heptane titration of the oil toinduce asphalt precipitation. Since the method required the addition ofsignificant amounts of n-heptane to the oil, it would be impractical toapply the test to an in-process stream and on a real-time basis.However, without the addition of n-heptane to initiate precipitation,the scattered acoustic energy measurements of the method taught bydeBoer, et al. would be meaningless.

Later, a group from the same laboratory reported the laboratory use ofthe same ultrasonic particle analyzer to study the utility of asphalteneinhibitors. (Bouts, et al., J. Petr. Tech., 782-787, September, 1995).The method included a test cell attached to a sonic probe which acted asdescribed above to measure the energy scattered from particles in theliquid. A multichannel analyzer counted particles and sorted thedetected scattered energy into thirteen amplitude classes. The twochannels measuring the smallest particles and the remaining 11 channelsmeasuring larger particles were respectively lumped together to monitor"small" and "large" particles versus time as the sample was titratedwith n-heptane to destabilize the asphaltenes. The purpose of the methodwas to test various inhibitors by monitoring the formation of asphalteneagglomerates as a function of the heptane added and the inhibitorcontent. The study did not show how to interpret or use particle sizedistribution data for more than two particle size ranges. Furthermore,the article did not disclose how the agglomerative state of asphalteneparticles in a hydrocarbon liquid could be determined on a real-timebasis, without dilution, or without removing a sample of the liquid fromthe stream or tank in which it is contained.

Thus, despite progress with promising techniques in related areas, asuitable method has not been available to measure the agglomerativestate of asphaltenes in oils, such as crude oil or any other opticallyopaque hydrocarbon liquid, on a real-time basis and without removing asample from the process stream. The lack of such method has also limitedthe ability to control the agglomeration of asphaltenes in such oils.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of the components of a system that may be usedto practice an embodiment of the present invention;

FIG. 2 shows a diagram of a focused acoustic probe mounted in a pipelinewherein liquid flow direction is indicated by the arrow and the focalregion of the acoustic signal is indicated by the intersection of thedashed lines;

FIG. 3 shows a diagram of a system which may be used in fieldapplications of the present invention wherein the acoustic probe isshown mounted in a retractable system;

FIG. 4 is a block diagram of a system that may be used in the presentinvention to acquire and transform the scattered acoustic signal into amagnitude vs. frequency format;

FIG. 5 is a magnitude vs. frequency plot of detected back-scatteredacoustic energy in an undiluted untreated crude oil within a frequencyrange of about 0.01 MHz to about 20 MHz;

FIG. 6 is a magnitude vs. frequency plot of detected back-scatteredacoustic energy within a frequency range of about 0.01 MHz to about 20MHz in an undiluted crude oil that has been treated to suppress theformation of asphaltene particles;

FIG. 7 is a magnitude vs. frequency plot of the data of FIG. 5 and FIG.6 illustrating one method of comparing magnitude vs. frequency data fromany oil or hydrocarbon liquid with a standard; and

FIG. 8 is a block diagram of a system that may be used to practice anembodiment of the present invention wherein the measurement of theagglomerative state of the asphaltenes in a hydrocarbon liquid initiatesan action to control agglomeration.

Corresponding reference characters indicate corresponding partsthroughout the several views of the drawings.

SUMMARY OF THE INVENTION

The present invention, therefore, is directed to an improved method formeasuring the agglomerative state of asphaltenes in a process flowstream of oil containing asphaltenes, comprising applying to the oil inthe process flow stream a signal of acoustic energy, thereby scatteringat least part of the energy; detecting the scattered acoustic energyover a selected frequency range; resolving the magnitude of the detectedscattered acoustic energy at selected frequencies within the selectedfrequency range; and determining the agglomerative state of theasphaltenes.

Further, the inventors provide a method for determining theagglomerative state of asphaltenes in an oil containing asphaltenescomprising removing a sample of the oil; applying to the oil a signal ofacoustic energy, thereby scattering at least part of the energy;detecting the magnitude of the scattered acoustic energy over a selectedfrequency range; resolving the magnitude of the detected scatteredacoustic energy at selected increments within the selected frequencyrange; deriving from such resolution a distribution of the relative sizeof asphaltene particles scattering acoustic energy within the selectedfrequency range; determining the agglomerative state of the asphalteneparticles; and returning the undiluted oil sample.

Also provided is a method for controlling the agglomeration ofasphaltenes in oil which comprises applying a signal of acoustic energyto the oil, thereby scattering at least a part of the energy; detectingthe scattered energy over a selected frequency range; resolving themagnitude of the detected scattered energy at selected increments withinthe selected frequency range; comparing the resolved detected scatteredenergy with a standard; and acting to control the number of particleshaving a particle size corresponding to the selected incrementalfrequencies.

Among the several advantages of this invention may be noted theprovision of a method for the determination of the agglomerative stateof asphaltenes in an oil, such as a crude oil or a process stream ofhydrocarbon liquid, on a real-time basis; the provision of such methodthat can be conducted in-line, without removing a sample from theprocess stream; and the provision of such method that facilitates thecontrol of the agglomeration of asphaltenes in oils.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In accordance with the present invention, it has been discovered thatthe concentration and distribution of relative sizes of asphalteneparticles, or "agglomerative state", of asphaltenes in a process flowstream of oil containing asphaltenes can be measured in real-time byapplying a signal of acoustic energy to the oil in the process flowstream, thereby causing at least some part of the signal to scatter asit encounters asphaltene particles within the oil, and detecting thescattered acoustic energy over a selected frequency range. The magnitudeof the detected scattered acoustic energy may be resolved at selectedfrequencies within the selected frequency range. If desired, adistribution of the relative size of asphaltene particles whichscattered acoustic energy within the selected frequency range may becalculated from the magnitude vs. frequency data by correlating themeasured data with a known standard or a model. Whether or not themagnitude vs. frequency data is correlated with a particle sizedistribution, the agglomerative state of the asphaltene particles in theoil may be determined by comparing the magnitude vs. frequency data, orthe particle size distribution, with a standard so that differencesbetween the two indicate the state of asphaltene particle agglomerationin the oil.

Actions for controlling asphaltene agglomeration in the oil then may bebased on such measurement. In one embodiment of the invention, moreover,the use of, for example, an oscilloscope and computer to resolve thedetected scattered energy quickly into magnitude at each of a largenumber of frequencies and to derive a relative particle sizedistribution permits the measurement to occur on a real-time basis.Also, because the determination of the agglomerative state of theasphaltene can be carried out without titration, or other modificationor dilution of the liquid being tested, the method can be used in-line,and without diluting, or otherwise contaminating, the process flowstream or any samples that may be taken.

In the present application, the term "oil" is meant to include crude oiland any other liquid hydrocarbon. While the method of the presentinvention may be used on any oil, it is more advantageously used on anoil which contains asphaltenes and is most advantageously used on oilsthat are optically opaque.

A system that may be used to practice the present invention is shown inthe block diagram of FIG. 1. In general, a pulse generator, or pulser30, generates an electrical signal which is transmitted to an acousticprobe 10 that transduces the electric signal into an acoustic signalthat is transmitted into the oil that is to be tested. The same probe,or, optionally, a separate sensor 20, detects an "echo" of the acousticsignal which is caused by scattering of the signal as it encountersasphaltene particles in the oil. An amplifier 40 amplifies the detectedscattered signal and transmits the amplified signal to an oscilloscope50 which converts the signal from analog to digital, selects that partof the detected scattered signal that results from scattering in thefocal region of the probe (which is termed "gating"), and transforms theamplitude vs. time signal into a magnitude vs. frequency distribution.This distribution is transmitted to a computer 60 that compares thedistribution with a standard and determines the agglomerative state ofthe asphaltenes in the oil.

As would be readily recognized by one of ordinary skill in the art ofultrasonic measurement, the system described above could be easilymodified while still carrying out the same functions. For example, theelectric signal to be transmitted to the probe 10 could be generated bythe oscilloscope 50, or a combined pulser/amplifier, as well as by thepulser 30. Alternatively, the oscilloscope 50 could provideamplification of the sensed signal and replace the separate amplifier40. Likewise, if desired, the computer 60 could accomplish thecalculation functions ascribed above to the oscilloscope 50. Othersystems for the ultrasonic measurement of particles in liquids aregenerally described in U.S. Pat. No. 4,412,451 to Uusitalo et al., U.S.Pat. No. 4,509,360 to Erwin et al., U.S. Pat. No. 4,706,509 to Riebel,and U.S. Pat. No. 5,121,629 to Alba.

The various parts of a system to practice the present invention andtheir operation are described as follows.

The acoustic probe 10 is a focused ultrasonic transducer that applies asignal of acoustic energy into the oil. The probe includes, in general,a piezoelectric crystal capable of transforming electrical signals intophysical pulses. If the crystal is in contact with a fluid, suchphysical pulses are transferred to the fluid and initiate waves having afrequency that is controlled by the frequency of the electronic signal.Preferably, the probe 10 also contains a lens to focus the signal asillustrated by the dashed lines of FIG. 2. The focal length of the probe10 is the distance from the end of the probe to the point where the wavepatterns converge. While focal length is not critical, it is preferablefor the focal length to be less than the distance from the lens of theprobe 10 to any opposing pipe or tank wall; i.e., the focal regionshould be in the fluid of interest. A probe 10 having a focal length ofabout 100 mm is suitable for the present invention as long as no wall,pipe or other process equipment structure intervenes between the probe10 and the focal region.

The acoustic probe 10 should be capable of sending an acoustic signalhaving a duration, amplitude and frequency range suitable for theinvention. Such signal may be a pulse or a "tone-burst".

If the signal is a pulse, it is transmitted to the probe 10 as ahigh-voltage spike of short duration and typically repeated many timesper second. For example, a 5 MHz probe which can transmit a 10nanosecond (ns), 300 volt signal is suitable for some applications ofthis invention. However, a 10 MHz probe is preferred, while a 100 MHzprobe is more preferred and a 200 MHz probe is most preferred.

If the signal is a tone-burst, it is directed into the oil in place ofthe spike, or pulse, just described. The tone-burst sweeps through thefrequency spectrum selected for use and each frequency is detected andanalyzed separately. The tone-burst will preferably have a duration ofbetween 4 and 8 cycles. The actual time duration will depend on theperiod (T) of the tone. If there are (n) cycles in the tone, then theduration will be nT.

Typical operation of an acoustic probe and ultrasonic systems similar tothose suitable for use in the present invention is described, forexample, by Urick, R. J., J. Appl. Phys, 18, 983-987 (1947); McClements,D. J., et al., J. Phys. D: Appl. Phys, 22, 38-47 (1989); Holmes, A. K.,et al., J. Coll. Int. Sci, 156, 261-268 (1993); McClements, D. J., Adv.Coll. Int. Sci, 37, 33-72 (1991); McClements, D. J., J. Acoust. Soc. Am,91, 849-853 (1992); Pinfield, V. J., et al., J. Coll. Int. Sci, 166,363-374 (1994) and McClements, D. J., The use of ultrasonics forcharacterizing fats and emulsions, Ph.D. Thesis, Food ScienceDepartment, University of Leeds, UK (1988); each of which is herebyincorporated by reference.

The probe should be capable of resisting temperature to 200° C., andpreferably to 300° C., and most preferably to 500° C. The probe shouldalso be capable of resisting pressure to about 10,000 Pa, and preferablyto about 100,000 Pa and most preferably to about 5,000,000 Pa. Moreover,the probe should preferably be capable of resisting chemical corrosionand physical erosion by the oils in which it is to be used.

Suitable probes may be commercially obtained or may be fabricated. Onetype of commercially available acoustic probe suitable for use in thepresent system is a barium-titinate ceramic 10 MHz probe as suppliedwith a Balteau-Sonatest UFD-1 ultrasonic tester.

The acoustic probe 10 may also be used as a sensor to detect acousticenergy scattered by particles in the oil. Alternately, a separate sensor20 may be used. If a separate sensor 20 is used, it may be placedanywhere in the fluid in relationship to the probe 10, but close enoughto receive energy scattered from the focal region. The sensor, whetherthe probe 10, or a separate sensor 20, converts the acoustic waves ofthe scattered acoustic energy encountering the piezoelectric crystal toan electrical signal. Operation of an acoustic energy sensor, ingeneral, is described by the references given above in the sectiondescribing the probe 10.

It is preferable that the sensor of the subject system has sensitivitysuitable for sensing back-scattered energy having frequencies up to 20MHz, but a probe which can sense back-scattered energy havingfrequencies up to 100 MHz probe is more preferable and a probe which cansense back-scattered energy having frequencies up to 200 MHz probe ismost preferable.

The acoustic probe 10 may be mounted in a retractable mounting, as shownin FIG. 3., for controlled insertion of the probe into a pipeline or atank. The retractable mounting permits the probe 10 to be easilywithdrawn from contact with the liquid for maintenance or replacementwithout dismantling the tank or line. The retractable mounting may bemanually activated, or may be motor driven. The design of the mountingthat is used for the probe 10 is not critical and any technician ofordinary skill would be able to design a suitable mounting.

A pulser 30 provides the necessary input signal to drive the transducerof the probe 10. As mentioned above, the oscilloscope 50 can also beused to generate the input signal.

An amplifier 40 amplifies the detected scattered signal before it istransmitted to the oscilloscope 50. The pulser 30 and amplifier 40 maybe separate components or may be combined in a single component. Forexample, a UTEX UTP320, from UTEX Scientific Instruments, Inc., providesboth pulser 30 and amplifier 40 functions in a combined component.

The oscilloscope 50 of the present system should be capable of gatingthe scattered acoustic signal, carrying out an analog to digitalconversion and, preferably, transforming the signal from an amplitudevs. time format to a magnitude vs. frequency format. The gating functionlimits the signal to the detected energy scattered by particles withinthe focal region (region "B" in FIG. 4). Thus, it deletes that part ofthe signal due to the input pulse (region "A" in FIG. 4), any reflectiondue to an opposing pipe wall and all other portions of the signal exceptfor the scatter due to particles in the focal region.

Oscilloscopes that are suitable for use in the present system are, forexample, a LeCroy Model 9450 and a LeCroy Model 9320. Such oscilloscopesshould be complete with software suitable for waveform processing suchas, for example, Waveform Processing Packages One and Two, availablefrom LeCroy. Data from the oscilloscope is transferred to a computer 60by using, for example, a National Instruments IEEE Plug and Playadapter.

A computer 60 is used in the present system to receive the signal fromthe oscilloscope 50 and to store the waveforms for future reference andalso to compare the waveforms against reference scans and otherstandards, to perform particles size modeling and analysis, to determinethe agglomerative state of the asphaltenes in the oil and to initiateany desired alarm or control action.

While the type and computational speed of the computer are not critical,it is preferred that a personal computer having a Pentium® processor, orits equivalent, and having spreadsheet software such as, for example,Microsoft Excel®, be used in the present system.

The several components of a system suitable for practicing the presentinvention should be interconnected as indicated in FIG. 1 and to othernecessary electrical sources or components to enable the proper andintended operation of each component. In general, the probe 10 should beconnected so as to receive an input electrical signal from the pulser 30and to transmit a detected scattered acoustic signal to the amplifier40. The amplifier 40 should be connected to transmit an amplifiedscattered acoustic signal to the oscilloscope 50. The oscilloscope 50 isconnected to the computer 60 so that, after performing necessary gatingand transforming steps, it can transmit data to the computer 60 forfurther calculations resulting in the determination of the agglomerativestate of the asphaltenes and the initiation of any desired alarm orcontrol functions.

The present method for measuring the agglomerative state of asphaltenesin a process flow stream of oil containing asphaltenes is carried out byapplying to the oil in the process flow stream a signal of acoustic, orultrasonic, energy. The present method is capable of determining theagglomerative state of asphaltenes in almost any oil that containsasphaltenes, but is especially useful for crude oil, other opticallyopaque streams, or streams where other in-line, real-time sensingmethods can not operate because of high pressures or high temperatures.

The acoustic signal is preferably applied to an oil for which theagglomerative state is to be determined before any dilution of the oilthat may be involved in the processing (e.g., refining) which the oil isto undergo and without diluting or adulterating the oil during theapplication of the signal. The acoustic probe 10 is most preferablyinstalled on the reactor, vessel, exchanger, pipeline, tank or othercontainer or conduit in which the oil is being stored, transferred, orprocessed so that the signal may be applied to the oil in the processflow stream and without removing the oil from such process flow stream,thereby avoiding disruption of or interference with the storage,transfer or process to which the oil is being subjected.

The term, "process flow stream", as it is used herein, means any streamor bulk amount of oil that is not a small sample and is meant to includeoils in pipelines, reactors, heat exchangers, tanks, pumps, pipes,lines, or any other container or conduit in which oil is conventionallystored, handled, processed, transported, or transferred to, from orduring processing or storage, but is not meant to include small samplesof oil which have, for testing purposes, been removed from or separatedfrom a bulk amount of the oil for which the agglomerative state ofasphaltenes is to be measured or controlled.

When it is said that a test, probe, or instrument is applied, "in-line",it means that the test, probe, or instrument of interest is applieddirectly in the process flow stream, rather than to a sample of suchstream.

The acoustic signal should be applied to the liquid as a pulse or acontinuing series of pulses, or as a tone-burst. The frequency range ofthe acoustic signals of most interest in the present method is roughlyfrom 0.1 MHz up to about 200 MHz.

Acoustic waves are reflected when they hit an interface between theliquid and a discontinuity such as a particle. As the acoustic signalpropagates through the liquid, it encounters whatever particles are inits path. When a focused acoustic probe is used, the acoustic signalconverges in a focal region creating a particularly strong signal atthat point. When the signal encounters a particle, some of the acousticenergy is scattered. Particles of different size cause energy to bescattered at different frequencies. Furthermore, the more particlespresent in the path of the signal, the greater the amount of theacoustic energy that is scattered. Therefore, a particular distributionof size and number of particles in a liquid results in scatteredacoustic energy characteristic for both the size and number of theparticles. The particle size distributions obtained from the acousticscattering technique correspond to the distribution of asphalteneparticles.

The scattered acoustic energy is detected by a sensor or detectordescribed above. While the probe 10 may also act as the sensor forback-scattered energy, separate sensors 20 may be placed relative to thesignal probe to detect forward scatter, or energy scattered at any otherangle. The detection of back-scattered energy has better potential foradaptation to an on-line device because, for example, one probe can actas both signal probe and sensor.

When asphaltenes agglomerate to form particles, the first particlesformed may be relatively small, perhaps comprising only a few asphaltenemolecules. However, if conditions persist which favor agglomeration, theasphaltene particles grow. It is important, therefore, that thescattered acoustic energy be detected in a frequency range that includesthe frequencies characteristic for energy scattered by the particularasphaltene particles in the oil. The inventors have discovered that thischaracteristic frequency range may vary with the type of oil and type ofprocess being imposed upon the oil. Measuring scattered energy over thewrong frequency range may entirely miss any asphaltene agglomerates thatform. Thus, a characteristic frequency range may have to be calibratedand optimized for each installation of the system. As long as one isaware of the need to determine a suitable frequency range, this can beeasily done by one skilled in the art without undue experimentation.

Detection of scattered acoustic energy over a frequency range of fromabout 0.1 MHz to about 20 MHz is suitable for determining the state ofagglomeration of asphaltenes in a typical oil, but, in order to insurethe detection of a wider range of particle sizes, detection over afrequency range of from about 0.1 MHz to about 100 MHz is morepreferable and detection over a frequency range of from about 0.1 MHz toabout 200 MHz is most preferable. However, once the frequency rangecharacteristic for energy scattered by the particular asphalteneparticles in a particular oil has been determined, it may be useful todetect scattered energy only over, or within, this more limited range.

If a spike, or pulse, input signal is used, the echo, or scatteredsignal, is detected as an amplitude vs. time plot resulting from eachsuch input pulse, as illustrated in FIG. 4. This plot of the amplitudeof the detected scattered signal received from each pulse vs. time showsthe detected signal as a spike at time=0, indicative of the input pulseitself, and, thereafter, the sensor detects acoustic energy scattered bymaterial in the path of the acoustic wave. The distance along the "time"axis corresponds to distance from the end of the probe because the timerequired for a wave to hit a particle and reflect back into the probe iscontrolled by the speed of sound in the oil and the distance of theparticle from the end of the probe. The amplitude of the detected energyis proportional to the concentration of the particles in the path of thesignal. A particularly strong response is received from the focal regiondue to convergence of the input signal at that point.

The signal received by the detector is amplified using a narrow bandamplifier and tuned to match the transducer fundamental frequency. Thesignal is gated to focus on scattering from the focal region and canthen be converted into a digital signal by an analog/digital converter.

The amplified, converted signal is then resolved into a magnitude vs.frequency format so that the relative particle size distribution may becorrelated and the state of agglomeration of the asphaltenes may bedetermined. Methods for carrying out this resolution are described byMcClements, D. J., et al., Ultrasonics, 31, 433-437 (1993); McClements,D. J., et al., J. Coll. Int. Sci., 160, 293-297 (1993); Dickenson, E.,et al., J. Coll. Int. Sci., 142, 103-110 (1991); and McClements, D. J.,and M. J. W. Povey, Ultrasonics, 30, 383-388 (1992); which referencesare hereby incorporated by reference.

The amplified, gated and converted signal is transformed from anamplitude vs. time format into a magnitude (decibels, dB) vs. frequency(MHz) format by Fourier transformation. The magnitude vs. frequency datais then averaged, preferably over about 200 pulses, and exported eitherto the oscilloscope or computer screen for display as a magnitude vs.frequency plot, as shown in FIG. 5 and FIG. 6, or to the computer forstorage and further computations.

Once the detected signal is resolved into magnitude versus frequencydata, a distribution of the relative size of asphaltene particlesscattering acoustic energy within the selected frequency range can bederived, if desired. This derivation may be done by correlating thefrequency, at any specific frequency within the frequency range ofinterest, with a particular known particle size. This may be done byobtaining the frequency response data for standard mixtures containingparticles of known sizes, or by prediction of a frequency vs. particlesize correlation model based on scattering theory. Comparison against aknown standard has been described by Povey, M. J. W., and M. G. Scanlon,J. Coll. Int. Sci., 93 (2), 565-566 (1983), which is hereby incorporatedby reference. Correlation of particle size with frequency by comparisonagainst a scattering theory model has been described by Pinfield, V. J.,et al., Ultrasonics, 33 (3), 243-251 (1995). This model may be improvedwith corrections for thermal scattering effects as described byPinfield, V. J. and M. J. W. Povey, J. Phys. Chem. B, 101, 1110-1112(1997), each of which references is incorporated by reference.

Alternately, the relative particle size distribution can be obtainedfrom data of amplitude and phase vs. frequency, as well as fromamplitude vs. time data.

The next step is to compare the magnitude vs. frequency data, or theparticle size distribution, against some standard and determine theagglomerative state of the asphaltenes.

Magnitude vs. frequency data represents a distribution of relativeparticle sizes, rather than absolute particle sizes, and distributionsof either relative or absolute particle sizes can be used as a measureof the agglomerative state of the particles in the oil. The distributionas measured in an oil is compared with a standard having a knownagglomerative state and the differences between the measureddistribution and the standard indicate the agglomerative state of themeasured oil. For example, the baseline at 0 dB magnitude as shown inFIG. 5 could serve as the standard with comparison against the measureddistribution from crude oil. The utility of such a simple standard isillustrated by the similarly flat distribution shown in FIG. 6 of crudeoil treated with an agglomeration inhibitor and having no particulates.The spikey signal in FIG. 5 between about 14 MHz and 20 MHz isattributed to asphaltene particulates in the crude oil.

The use as a standard of a crude oil treated with an agglomerationinhibitor is shown in FIG. 7, where the magnitude vs. frequencydistribution of the standard is superimposed over the same data for acrude oil. As above, the differences in the two signals in the frequencyrange between about 14 MHz and about 20 MHz indicate the presence ofagglomerates in the crude.

Since the operation of the acoustic and electronic steps of the presentinvention take place very quickly and because no step requires waitingfor a change to occur in the liquid (as in the methods disclosed byBouts, deBoer and Anfindsen), or requires the titration or addition ofany material to the liquid (as in Bouts and deBoer), the determinationof agglomerative state takes place substantially instantaneously and canbe advantageously applied on a real-time basis to process steams and thelike. In the present case, substantially instantaneously means that themeasurement can be made in less than about one second. It is understood,however, that the system applying the present method may take suchmeasurements continually and, if desirable, can average the results ofseveral measurements taken over any period of time desired.

In another embodiment of the present invention, a liquid sample may beremoved or conducted from the container in which the oil on which theagglomerative state of asphaltenes is to be determined. The same stepsof applying acoustic energy, detecting scattered energy and resolving,deriving and determining the agglomerative state of the asphaltenes arecarried out as described above, except that the applying and sensingtakes place in a flow cell, or measurement cell, rather than in theprocess or tank. After such measurements are complete, the undiluted,unadulterated oil sample is returned.

An additional embodiment of the present invention is a method forcontrolling the agglomeration of asphaltenes in oil. A system such asthat shown in FIG. 8 can be used for such control function. The subjectmethod involves the steps of applying a signal of acoustic energy to theoil and detecting the scattered energy over a selected frequency asdescribed above. The detected energy is then resolved into its magnitudeat selected increments within the selected frequency range. Themagnitude of the detected scattered energy is related to the number ofparticles of a certain size and, for control purposes, it is notnecessary to derive the distribution of the relative particle sizes, butonly to compare the magnitude of the detected signal at one or moreselected frequencies with a standard, in order to make a decision to actto control the number of particles corresponding to the particularfrequency or frequencies.

As an application of the present invention is calibrated and tuned for aparticular application, it may be desirable to limit the frequency rangeselected for detecting scattered acoustic energy to the frequency rangescattered by the agglomerated asphaltene particles characteristic of theoil. Preferably, the frequency range selected for detecting scatteredacoustic energy is from about 14 MHz to about 20 MHz.

The act to control the number of particles may take any number of forms.Often the control will take the form of coordinating a dose or series ofdoses of agglomeration inhibitor to the agglomerative state of thefluid. For example, the device may cause a change in process conditionssuch as flow rate, temperature, or pressure, or may cause the additionof an agglomeration inhibitor, a surfactant, or other chemical additive.Alternately, the control act may be to divert a stream, or slow down,speed up or stop a process. A variety of techniques for control ofasphaltene agglomeration are well known to those of ordinary skill inthe art.

Use of the method for measuring the state of agglomeration in a controlloop is shown in FIG. 8. The control loop can include the computer 60and a control actuator 70, with the feedback data provided by a systemapplying the method for measuring the agglomerative state of the presentinvention.

Several particularly useful features are provided by the method ofmeasuring the agglomerative state of asphaltenes in an oil. Themeasurement can be carried out on a real-time basis since there are notitrations or other additions to the oil and, thus, no waiting forchanges induced by such additions. The application, detection andresolution of the acoustic signal is carried out very rapidly. Even ifthe detected scattered energy signal is averaged over 100 separatepulses, the relative particle size distribution can be derived and theagglomerative state of the asphaltenes determined in a time of wellunder one second.

The acoustic probe may be installed directly into a tank or linecontaining the bulk oil and the measurement may be taken on an in-linebasis. No sampling is necessary. Because the test is non-diluative, if asample is taken, it may be returned to the bulk oil, if desired, withoutdilution or adulteration of the oil.

Furthermore, because of the simple and resistant nature and materials ofthe acoustic probe, the method may be used in streams and underconditions where other methods would fail due to temperature, pressure,or corrosiveness of the oil.

Because of the features described above, the method for controllingagglomeration of asphaltenes in oil has the useful feature of being areal-time control technique. A signal from the measuring systemsubstantially reflects the state of the oil in a process flow stream ata given time. Based on that information, actions can be taken to controlthe agglomeration of the asphaltenes and the results of those actionsmay be monitored without unacceptable time lag. A feedback control loophaving a time-lag constant within normal practice is then made possible.This permits the use of conventional industrial control equipment toeffect the process changes called for by the controller (in this case,the personal computer).

Industrial Application

The measuring and control systems of the present invention may be usedfor any in-line, in-tank, or in-process application requiring themeasurement or control of the agglomerative state of asphaltenes in oil.

One potentially useful application is during production at the wellhead,or in the borehole, where changes in temperature and pressure oftenresult in precipitation of asphaltenes from crude. This results inplugging lines and equipment and forces periodic shutdowns andmaintenance to remove the asphaltene solids. Installation of a measuringand control device based on the present invention would permit additionof, for example, asphaltene precipitation inhibitors on an as-neededbasis in order to minimize asphaltene agglomeration or in reduced orotherwise adjusted doses in coordination with the agglomerative state tooptimize treatment. This is an improvement over the addition of suchexpensive chemicals on a continuous basis because, in general, less ofthe inhibitor is used over time and prevention of plugging permitslonger periods of operation.

The methods of the subject invention may also be applied to in-processstreams, such as visbreaker streams, where hot, opaque liquids rule outmost conventional particle sensors.

The following example describes a preferred embodiment of the invention.Other embodiments within the scope of the claims herein will be apparentto one skilled in the art from consideration of the specification orpractice of the invention as disclosed herein. It is intended that thespecification, together with the example, be considered exemplary only,with the scope and spirit of the invention being indicated by the claimswhich follow the example.

EXAMPLE 1

Measurement of the state of agglomeration of asphaltenes in crude oil bya pulse-echo technique.

A system including a UTEX UTP320 pulser/amplifier was used to provide apulsed electrical signal to a 10 MHz focused ultrasonic probe. The tipof the probe was submerged in a sample of undiluted crude oil. The pulsesignal used in this example had the following characteristics:

Voltage=1 kV (1,000 volts)

Duration=3 nanoseconds

Impedance=70 Ohms

Electrical power=14,285.7 watts

Efficiency=0.1

Acoustic power=1,428.57 watts

Spike frequency=1,000 Hz

On/Off Ratio=3×10⁻⁶

Mean Electrical Power=0.04286 watts

Mean Acoustic Power=0.00429 watts

The probe also acted as the sensor to detect back-scattered energy. TheUTEX pulser/amplifier amplified the detected signal and transmitted itto a LeCroy Model 9320 oscilloscope. The oscilloscope carried out ananalog-to-digital conversion of the signal and gated the signal toback-scatter by particles in the focal region. The gate width wasapproximately twice the signal duration (i.e., 2 nT; corresponding to awidth of about 3000(nT) in oil). The oscilloscope then carried out aFourier transform of the signal to convert it from an amplitude vs. timeformat to a magnitude vs. frequency format. The magnitude vs. frequencydata was averaged over 200 scans prior to being exported to a personalcomputer for filing and further computations in an Excel® spreadsheetprogram. A plot of the resulting magnitude vs. frequency data is shownin FIG. 5 over a frequency range of approximately 0.1-20 MHz . A solidline showing an average value for these data points is also included onthe plot.

The same type of measurement was carried out on a sample of the same oilas used above, but to which a chemical inhibitor had been added toprevent asphaltene agglomeration. The magnitude vs. frequency plot forthis inhibited sample is shown in FIG. 6.

The computer compared data from the oil (as shown in FIG. 5), with thestandard (as shown in FIG. 6), by superimposing one over the other asshown in FIG. 7. Alternately, the plot of FIG. 5 could simply becompared with an arbitrary baseline, such as the "0 magnitude" baselineshown in FIG. 5, or with a model, or any other standard against whichthe state of agglomeration of the unknown could be determined.

In FIG. 7, the trace of the asphaltene particles scattering primarily inthe 14 MHz-20 MHz region can be contrasted with the lack of such scatterin the inhibited sample of the same oil. This difference represents thedegree of asphaltene agglomeration in the crude oil.

In view of the above, it will be seen that the several advantages of theinvention are achieved and other advantageous results attained.

As various changes could be made in the above methods and compositionswithout departing from the scope of the invention, it is intended thatall matter contained in the above description shall be interpreted asillustrative and not in a limiting sense.

What is claimed is:
 1. A method for measuring the agglomerative state ofasphaltenes in a process flow stream of oil containing asphaltenes,comprising applying to the oil in the process flow stream a signal ofacoustic energy, thereby scattering at least part of the energy;detecting the scattered acoustic energy over a selected frequency range;resolving the magnitude of the detected scattered acoustic energy atselected frequencies within the selected frequency range; anddetermining the agglomerative state of the asphaltenes.
 2. A method asset forth in claim 1 wherein the selected frequencies within theselected frequency range comprise at least three different frequencies.3. A method as set forth in claim 1 wherein the selected frequencieswithin the selected frequency range comprise at least fifteen differentfrequencies.
 4. A method as set forth in claim 1 wherein the steps arecarried out without diluting the hydrocarbon liquid.
 5. A method as setforth in claim 1 wherein the steps of the method are carried outsubstantially instantaneously.
 6. A method as set forth in claim 1,wherein the detected scattered acoustic energy is back-scatteredacoustic energy.
 7. A method as set forth in claim 1, wherein theselected frequency range is from about 0.1 MHz to about 20 MHz.
 8. Amethod as set forth in claim 1, wherein the selected frequency range isfrom about 0.1 MHz to about 200 MHz.
 9. A method as set forth in claim1, wherein the selected frequency range is from about 14 MHz to about 20MHz.
 10. A method as set forth in claim 1, wherein the detecting iscarried out by at least one sensor which sensor is incorporated in asignal input probe.
 11. A method as set forth in claim 1, wherein thedetecting is carried out by at least one sensor which sensor is separatefrom a signal input probe.
 12. A method as set forth in claim 1, whereinthe signal of acoustic energy is applied as a pulse and the step ofresolving the magnitude of the detected scattered acoustic energy atselected frequencies within the selected frequency range comprisesgating the detected scattered acoustic energy to that part of thedetected energy emanating from a focal region and Fourier transformingthe detected scattered energy into a magnitude vs. frequency format. 13.A method as set forth in claim 1, wherein the signal of acoustic energyis applied as a tone-burst and the step of resolving the magnitude ofthe detected scattered acoustic energy at selected frequencies withinthe selected frequency range comprises detecting the magnitude of thescattered energy at selected frequencies within the selected frequencyrange.
 14. A method as set forth in claim 1, wherein determining theagglomerative state of asphaltene particles having a size distributionis effected by comparing the size distribution of the asphalteneparticles scattering acoustic energy within the selected frequency rangewith a standard.
 15. A method as set forth in claim 14, wherein thestandard is a sample of known particle size.
 16. A method as set forthin claim 14, wherein the standard is a model of particle size based onscattering theory.
 17. A method for measuring the agglomerative state ofasphaltenes in an oil containing asphaltenes comprising:a. removing asample of the oil, and without diluting said sample; b. applying to theoil a signal of acoustic energy, thereby scattering at least part of theenergy; c. detecting the magnitude of the scattered acoustic energy overa selected frequency range; d. resolving the magnitude of the detectedscattered acoustic energy at selected increments within the selectedfrequency range; e. deriving from such resolution a distribution of therelative size of asphaltene particles scattering acoustic energy withinthe selected frequency range; f. determining the agglomerative state ofthe asphaltene particles; and g. returning the undiluted oil sample. 18.A method for controlling the agglomeration of asphaltenes in oil whichcomprises applying a signal of acoustic energy to the oil, therebyscattering at least a part of the energy; detecting the scattered energyover a selected frequency range; resolving the magnitude of the detectedscattered energy at selected increments within the selected frequencyrange; comparing the resolved detected scattered energy with a standard;and acting to control the number of particles having a particle sizecorresponding to the selected incremental frequencies.
 19. A method asset forth in claim 18, wherein the frequency range selected fordetecting scattered acoustic energy is limited to a frequency range ofacoustic energy scattered by the agglomerated asphaltene particlescharacteristic of the oil.
 20. A method as set forth in claim 18,wherein the selected frequency range is from about 14 MHz to about 20MHz.